1. Field of the Invention
The invention relates to a backing of a card carrying electronic components, the backing offering low heat resistance so making it possible easily to dissipate a quantity of accumulated heat given off by the components, in the direction of a chassis which functions as a heat sink.
2. Description of Related Art
Usually, electronic components are mounted on cards comprising printed circuits which interconnect the said components. Generally, these cards are provided with electrical connection devices which make it possible to plug them into open or closed metal chassis (or racks) to form a casing, using likewise metal tracks on which they run. They are obviously electrically insulating but are sufficiently good conductors of heat.
When the components are giving off heat, this must be dissipated so that they are not damaged. The temperature which must not be exceeded is approx. 125.degree. C. in order to avoid any increase in the breakdown risk level (more generally known by the term "mean time between failure" or "MTBF").
When the cards are of epoxy resin or a glass fibre/epoxy resin complex or any other similar material which is not a good conductor of heat, the heat may be dissipated by means of thermal drains which link the components with the card supporting tracks or slides, the tracks being rigid with the chassis which acts as a heat sink or radiator.
Such a device is inadequate when the quantities of heat to be dissipated increase. Therefore, it has been suggested to use cards of a more elaborate construction. Usually, these cards comprise two parallel and flat outer plates constituting two printed circuit boards proper or two ceramic inter-connecting circuits (which will hereinafter be referred to likewise by the term `printed board`) on which the electronic components are mounted; they enclose a likewise flat metallic carrier which is a good conductor of heat.
Printed circuit boards are electrically insulating but must however offer an acceptable level of heat conductivity. Therefore, thin sheets of sintered alumina are usually chosen as the raw material for printed circuit boards while as a carrier material, the choice falls to co-laminated metal sandwiches of the Cu--invar--Cu or Cu--Mo--Cu type, which have the same expansion coefficient as the alumina.
The heat dissipation capacity of this device may be enhanced by causing a cooling fluid to circulate through the metallic carrier. But this means that, over and above the electrical connection means for the printed circuit board, it is necessary to provide hydraulic connecting means for the cooling circuits between the metallic carrier of the card or circuit board and the chassis; therefore, the hydraulic connections are mixed up with the electrical connections.
From the document IEEE 1985, National Aerospace and Electronics Conference (Vol. 2, 20th May 1985, Dayton, U.S., pp. 1244-47) it is also known to have a circuit board carrier consisting of a plurality of elementary heat conductive enclosures contained in a honeycomb-type casing closed by plates. Such a structure is not sufficiently homogeneous and may give rise to the risks of hot points due to the presence of the elements constituting the framework.
It can be seen that the amount of heat which can be dissipated is limited by the thermal resistance of the chain passing from the electronic components to the chassis, this latter obviously having sufficient capacity to dissipate whatever heat is conveyed to it.